1 Field of the Invention
The present invention relates to a color correction apparatus for effecting the color correction of an input/output apparatus connected to a system for inputting and outputting a color image, such as a color printing machine, a color printer, a color copying machine, a color facsimile machine, and a color display.
2 Description of the Related Art
Conventionally, the following apparatuses are known as color correction apparatuses for effecting color correction into input/output color representation values which are peculiar to an input/output apparatus connected to a color-image input/output system, such as a color printing machine, a color printer, a color copying machine, a color facsimile machine, and a color display.
Japanese Patent Application Laid-Open (JP-A) No. 6-296231 discloses a technique wherein, in an apparatus for transforming color representations, which are independent of characteristics of an input/output apparatus, such as L*a*b* values and L*u*v* values, into color representations for the input/output apparatus, such as CMY values and CMYK values by an interpolated lookup table method, the so-called affine transformation is performed with respect to the color representations which are independent of the characteristics of the input/output apparatus in an early stage of the retrieval of the lookup table.
In addition, Japanese Patent Application Laid-Open (JP-A) No. 6-334853 discloses a technique wherein the type of an input/output apparatus connected to an input/output system is determined, a color correction table corresponding to the type of apparatus is set in a memory, and color correction is executed with respect to inputted color image data on the basis of the color correction table. According to this technique, it is possible to correct with high accuracy those variations in the input signal and variations in the printed-output density that are ascribable to the type of apparatus.
In addition, with a color printing machine which makes use of a conventional rotary press or the like, color printed matter based on the so-called halftone images is prepared. Before preparing the color printed matter, a color printing proof image (also called a color printing proof is prepared in advance by using a color printer having a simple configuration, so as to proof the color printing on the basis of the image. By the use of this color printer, it becomes unnecessary to prepare a platemaking film, a printing-plate (PS plate), or the like concerning the color printing machine at the time of proofing, thereby making it possible to substantially improve the efficiency in the proofing operation.
Before preparing the color printing proof image for proofing, it is necessary to correct in advance the difference in density between the color printed matter and the printed output, which is attributable to printer conditions, such as the machine difference, changes over time, and the like, of the color printer (this correction is referred to as the calibration adjustment).
For example, as shown in FIG. 12A, even in the case of a printer in which the relationship between the output density and the printer signal inputted to an output unit is expressed by an output density curve 140 having reference gradations indicated by the chain line, its characteristic curve changes to an output density curve that is different from the output density curve 140, e.g., to an output density curve 142 indicated by the solid line, due to the difference of the individual apparatuses or because of the lapse of time. In this case, even if a printer signal P1 or P2 is inputted to the output unit of the relevant printer in an attempt to obtain an output density D1 or D2, the actually outputted density becomes D1xe2x80x2 or D2xe2x80x2, with the result that, if this state is left as it is, it is impossible to output an appropriate color printing proof image.
Accordingly, a printer signal P is converted into a signal Pxe2x80x2 on the basis of a conversion curve 150 shown in FIG. 12B, for instance, and the signal after correction is inputted to the output unit of the printer, thereby obtaining an output density at the reference gradation. With this conversion curve 150, the printer signals P1 and P2 before correction become signals P1xe2x80x2 and P2xe2x80x2 after correction. Hence, even with the output unit having the characteristic of the output density curve 142, if the printer signals P1xe2x80x2 and P2xe2x80x2 are inputted thereto, it is possible to obtain appropriate output densities D1 and D2, as shown in FIG. 12A.
In this calibration adjustment, a method is adopted in which, after the dot percent data of four plates for the respective colors, cyan (C), magenta (M), yellow (Y), and black (K), are converted by a four-dimensional (4D) conversion table for color correction incorporated in the color printer as image data, data converted by a one-dimensional (1D) conversion table for calibration are then printed out, and a comparison is made between the printed-out densities and the densities of the respective colors of a calibration chart (color patches) serving as a reference, so as to adjust the 1D conversion table for calibration. Namely, the image data are converted by tables in at least two stages.
Actually, in a color printer for color proofing, color correction is carried out by using tables in three stages by further adding a printing-condition correction 1D conversion table for correcting the printing conditions of the above two tables, as shown in FIG. 11, by taking into consideration the difference in printing conditions (e.g., the type of printing paper and the type of printing ink) concerning the color printing machine.
However, with the technique disclosed in Japanese Patent Application Laid-Open (JP-A) No. 6-296231, when color transformation is effected in two stages by the transformation using a lookup table and the affine transformation, there arises the problem that the processing speed of color transformation declines, and the apparatus becomes complex due to the, preparation of tables in two stages.
In addition, with the technique disclosed in Japanese Patent Application Laid-Open (JP-A) No. 6-334853, although improvement can be made on variations such as the output density due to the machine type of the input/output apparatus, in a case where color correction is effected by using a color correction table in one stage by concurrently taking into consideration other conditions such as printing conditions, it is necessary to prepare color correction tables, for each combination of various different kinds of conditions. Hence, there arises the problem that a memory of a very large capacity is required.
Furthermore, with the aforementioned conventional color printer as the apparatus for preparing a color printing proof image, since color correction is effected in three stages by using the 4D conversion table for color correction, the 1D conversion table for calibration, and the printing-condition correcting table, there arises the problem that the processing speed of color correction declines, and the apparatus becomes complex due to the preparation of tables in three stages.
In view of the above-described circumstances, it is an object of the present invention to provide a color correction apparatus with a simple configuration capable of effecting color correction calculation at high speed and with high accuracy.
To attain the above object, in accordance with a first aspect of the present invention, there is provided a color correction apparatus for effecting a plurality of kinds of color correction, including color correction as standard color transformation and color correction for correcting a difference in color output density due to at least one of a printer condition and a printing condition, comprising: input means for inputting color image data; color-correction calculating means for effecting the plurality of kinds of color correction of the color image data inputted to said input means, in one stage of synthesized color correction; and output means for outputting the color image data subjected to color correction by said color-correction calculating means.
In accordance with the above-described first aspect of the invention, the plurality of kinds of color correction, including color correction as standard color transformation and color correction for correcting a difference in color output density due to at least one of a printer condition and a printing condition are effected by the color-correction calculating means in one stage of synthesized color correction with respect to the color image data inputted to said input means. Then, the output means outputs the color image data subjected to color correction by the color-correction calculating means. Here, the printer condition refers to a condition on the output apparatus side, such as a machine difference of the output apparatus, a change in the environment in which the apparatus is placed, or a change over time. The printing condition refers to a condition on the color printing machine side, such as the type of printing paper and a printing environment for outputting a color print image. In addition, the standard color transformation refers to standard color correction which is effected in correspondence with an output system, and the like, of a color printer or a color printing machine, and is independent of the above-described individual conditions.
Thus, since a plurality of kinds of color correction are effected in one stage of synthesized color correction, it is possible to effect color correction with a simple configuration and at high speed as compared with a case where the plurality of kinds of color correction are effected in two or more stages. Moreover, since the difference in color ascribable to at least one of the: printer conditions and the printing conditions can be corrected, it is possible to obtain a color image with high accuracy. It should be noted that the color-correction calculating means may comprise, for example, a neural network which has undergone learning so as to effect the aforementioned plurality of kinds of color correction.
A second aspect of the present invention, comprise the above-described first aspect of the invention, wherein the color-correction calculating means effects the synthesized color correction on the basis of a single lookup table.
In accordance with the above-described second aspect of the invention, since synthesized color correction is effected on the basis of a single lookup table, it is possible to effect color correction with a simpler apparatus and at higher speed and with higher accuracy. Incidentally, interpolation calculation may be performed at the time of the synthesized color correction.
In accordance with a third aspect of the present invention, the color correction apparatus in accordance with the first or second aspect of the invention further comprises: storage means for storing for each kind of that color correction, a plurality of pieces of data for color correction that serves as a basis of the color correction; designating means for designating one piece of the data for color correction at a time for each kind of the color correction; and synthesizing means for preparing synthesized color-correction data that serves as a basis of synthesized color correction by combining the data for color correction designated by said designating means.
In accordance with the above-described third aspect of the invention, the storage means stores for each kind of color correction a plurality of pieces of data for color correction that serves as a basis of the color correction. The designating means designates one piece of the stored data for color correction at a time, for each kind of the color correction. Then, the synthesizing means prepares synthesized color-correction data that serves as a basis of synthesized color correction by combining the data for color correction designated by said designating means. The color-correction calculating means effects synthesized color correction on the basis of the synthesized color-correction data that has been synthesized. Thus, in accordance with this aspect of the invention, since synthesized color-correction data can be synthesized by designating data for color correction for each condition, it is possible to save memory and simplify the apparatus even in a case where color correction is effected with high accuracy by color correction that takes a large number of conditions into consideration.
The above and other objects, features and advantages of the present invention will become more apparent from the following detailed description of the invention when read in conjunction with the accompanying drawings.